So what is the difference with, say, very fine sand/dust? Density? E.g. do the same critters ingest fine sand - and if so, why plastic is more harmful? Also, if these microbeads are chemically inert, what is the harm from them entering the foodchain - as opposed, again, to e.g. dust?

Sand and dust tends to settle, where as these microbeads form a "soup".

The beads absorb some chemicals, and so they're not inert by the time they're eaten.

Do they decay in UV light? I know most plastic will tend to break down over time if left outside.